The invention relates to the field of direct thermal printing and especially to media used for such printing.
Direct thermal printable media is most widely used as facsimile paper but is also used in printers and in other applications requiring permanent imaging such as tags, tickets, and labels. In contrast to printing technologies that involve the transfer of ink from one location to another, direct thermal printing uses a special printable media that incorporates a color developing mechanism. Images are formed by exposing the color developing mechanism to concentrations of heat that produce localized chemical reactions involving a change of color (usually light to dark). The color developing mechanism is provided by a thermosensitive imaging material containing heat reactive chemicals such as leuco dyes or metallic salts.
Since direct thermal printing only involves the transfer of heat, printing of direct thermal media is simple and clean. Other advantages include low cost, low noise, and high speed. However, the thermosensitive imaging material is susceptible to damage from exposure to various environmental conditions including abrasion, heat, light, and chemicals such as oils, fats, blood, alcohol, solvent, and water. Much of this damage can be limited by applying protective coatings that block unwanted environmental interactions with the thermosensitive imaging material.
For example, U.S. Pat. No. 4,711,874 to Yuyama et al. suggests use of a water-soluble polymeric material as a protective overcoating for providing physical and chemical resistance. U.S. Pat. No. 4,717,709 to Suzuki suggests use of a polyolefin resin for similar purposes. U.S. Pat. No. 4,886,774 to Doi discloses a protective overcoating containing UV blockers. U.S. Pat. No. 5,286,703 to Wachi et al. discloses use of multiple protective overcoat layers including a first layer of water-soluble or water-insoluble polymers for chemical resistance and a second layer containing UV blockers.
The thermosensitive imaging material is applied as a coating to a substrate surface. Printing takes place by exposing the coating to a pattern of heat conducted from a thermal print head located adjacent to the substrate surface containing the coating of thermosensitive imaging material. The coating takes the form of the substrate surface including any irregularities or roughness in the surface. Accordingly, any surface irregularities in the thermosensitive coating vary spacing between the thermal print head and different points on the coating, causing unwanted dissipations of heat that interfere with image quality.
Two solutions are known to limit irregularities in the thermosensitive coating. One is to use only substrates with smooth surfaces. The other is to apply an undercoating between the substrate and the thermosensitive coating. The undercoating covers irregularities in the substrate surface and provides a smooth base for applying the thermosensitive coating. An example is found in U.S. Pat. No. 4,711,874 to Yuyama et al.
Protective overcoatings shield thermosensitive coatings from environmental interactions, and undercoatings provide a smooth base for applying thermosensitive coatings; but both add cost and complexity to direct thermal printable media. Also, the undercoatings can change the appearance or other desired properties of the substrates. However, without an undercoating, the choice of substrate is even more limited.
My invention obviates the need for both overcoatings and undercoatings that separately protect and support coatings of thermosensitive imaging material in direct thermal printable media. Coatings of thermosensitive imaging material can be used with a wider variety of underlying substrates without resort to undercoatings, because surface features of the thermosensitive coating are no longer linked to the surface features of the underlying substrates.
An example of my invention as a direct thermal printable laminate includes two adjacent substrates, one of which is preferably an optically and thermally transmissive film. A thermosensitive imaging layer is located adjacent to a back surface of the transmissive film, and the back surface of the transmissive film is bonded to a front surface of the other substrate for laminating the transmissive film to the other substrate. The thermosensitive imaging layer is reactive to transmissions of heat through the film (which is thermally transmissive) for forming images within the thermosensitive imaging layer that are visible through the same film (which is also optically transmissive).
Preferably, the thermosensitive imaging layer is bonded directly to the back surface of the film and indirectly to the front surface of the other substrate through the intermediacy of an adhesive layer. Accordingly, the thermosensitive imaging layer takes the form of the back surface of the film rather than the form of the front surface of the other substrate.
In addition to functioning as a transmitter of both light and heat, the film can perform two other main functionsxe2x80x94first, as a protective covering for the thermosensitive imaging layer and second, as a base for giving desired form to the thermosensitive imaging material. Thus, my invention permits many more materials to be used as substrates for direct thermal printing while protecting the thermosensitive imaging layer from unwanted environmental interactions.
My new laminate is printable by applying concentrations of heat from a thermal print head to the front surface of the film resulting in the formation of images within the thermosensitive imaging layer. The film is preferably much thinner in thickness than the other substrate to more efficiently transmit heat from the thermal print head to the thermosensitive imaging layer. Thicknesses less than 10 microns are preferred.
Another example of my invention as self-wound direct thermal printable tape includes a similarly transmissive film. A release layer is applied to a front surface of the film, and a thermosensitive imaging layer is applied to a back surface of the film. The thermosensitive imaging layer is reactive to transmissions of heat through the film for forming images within the thermosensitive, imaging layer. Adhesive layers are applied to front and back surfaces of a substrate. A first of the adhesive layers bonds the substrate to the film, and a second of the adhesive layers is intended for bonding the film and substrate to another substrate or object. However, prior to bonding the film and substrate to another substrate or object, the film and substrate are unwindable from a roll in which the second adhesive layer is in contact with the release layer.
Again, a wider selection of substrates can be used with my new direct thermal printable tape, because the transmissive film provides the thermosensitive imaging layer with a desired form as well as a protective covering against exposure to environmental hazards. The release layer on the front surface of the film eliminates any need for a separate release liner to protect the second adhesive layer prior to use. Also, since the thermosensitive imaging material is first coated on the film rather than on the underlying substrate, smaller production runs are more economical because the coated film can be divided for runs with different substrates.
In fact, a thin thermally transmissive film coated with a thermosensitive imaging material in accordance with my invention can be made as either an intermediate product intended for lamination to another substrate or as a final product that can be imaged prior to or in conjunction with its use. Preferably, the film is both thermally and optically transmissive and has a thickness no greater than 10 microns.